Former MI5 agent David Shayler is due in court at the start of one of the most explosive spy trials of the last 20 years.
Mr Shayler, 35, who faces three charges under the UK's Official Secrets Act, is accused of leaking details of the security services' activities to a Sunday newspaper four years ago.
He claimed agents in the 1970s tapped the telephone of Peter Mandelson and kept a file on British Home Secretary Jack Straw.
Mr Shayler says these disclosures, and a number he has made since then which are not subject to the trial, were made to expose incompetence and malpractice in the security services.
Arguing he is a genuine "whistleblower" exposing wrongdoing, Mr Shayler will claim his prosecution under the Official Secrets Act is incompatible with the Human Rights Act, which protects free speech.
Two of the charges relate to unauthorised disclosure of security service information and the third relates to revealing details of telephone taps. Each carries a maximum sentence of two years.
Whatever the outcome of the case, the Shayler affair is expected to have major repercussions on the Official Secrets Act.
The first few days of the trial, at the High Court in London, will be taken up with legal argument centring on various aspects of the two Acts.
There is a strong likelihood the case will get no further than the legal argument stage. Both the prosecution and the defence could take Mr Justice Moses's decision on the human rights issue to judicial review, delaying the case for a year or more.
PA